Shearer loaders use rack and pinion arrangements for advancing the shearer loader that are usually mounted on the goaf side on a scraper chain conveyor which is arranged and installed in the underground face, thus allowing the shearer loader, as the extraction machine, to be supported on the carrier bars by means of guide shoes and at the same time to guide on the guide rods. The teeth of the toothed wheels of the travel drive engage with the tooth gaps between the rack and pinion teeth in order to convert the rotatory movement of the toothed wheel into a translatory movement of the shearer loader. The rack and pinion arrangement is composed usually of rack and pinion rods, the length of which corresponds substantially to the length of each trough pan of the scraper chain conveyor, thus allowing the scraper chain conveyor, and to this extent also the machine track along with the rack and pinion, to adapt to an undulating course of the extraction face with depressions, saddles and curves. The construction of the rack and pinion arrangement, which is segment-like on account of the individual rack and pinion rods, allows a shearer loader to follow even a curved course of a face conveyor and also the horizontal and/or vertical kinks without becoming blocked.
During operational use of shearer loaders, the rack and pinion rods and also the rack and pinion arrangement as a whole are subject to considerable stresses. Although a fixed pitch dimension between the individual rack and pinion teeth can be ensured within a rack and pinion rod, the vertical and/or horizontal kinking movements of the individual rack and pinion rods, the curved course of the face and the like can lead, in particular at the joining points of adjacent rack and pinion rods, to agitated running of the machine and also to increased stressing of the rack and pinion rods and also the teeth of the driving sprockets.
A rack and pinion arrangement of the type in question with associated rack and pinion rods is known from DE 197 46 360 A1 in the name of the applicant. The known rack and pinion arrangement is successfully used by the applicant under the brand name “JUMBOTRACK” or “JUMBOTRACK 2000” as a system for advancing a shearer loader for extracting coal in underground mining. In the generic rack and pinion arrangement according to DE 197 46 360 A1, to improve the advancement of the shearer loader and also the tooth engagement ratio between the teeth of the driving sprocket and the rack and pinion teeth, the individual rack and pinion teeth have been given a particular tooth profiling which is characterized in that the tooth flanks of adjacent rack and pinion teeth, which tooth flanks diverge from one another towards the tooth head, have a relatively flat flank surface extending so as to be inclined at an angle of from about 8° to 15°, preferably 10° to 12° to the tooth center plane of the individual rack and pinion teeth. The fact that the tooth flanks of the rack and pinion teeth are designed as flat and steeply positioned surfaces allows raising of the extraction machine on account of transverse force components and also the interfering influences resulting therefrom to be to a large extent eliminated. In order at the same time to control problems at the joining points between adjacent rack and pinion rods, use is made, at both ends of the rack and pinion rods of the generic rack and pinion arrangement, of end teeth which are formed unsymmetrically based on their vertical end tooth center plane, the asymmetry being achieved in the generic rack and pinion rods in that the tooth flank of the end teeth that is positioned facing the inner rack and pinion teeth receives in each case a tooth flank extending at a steeper angle relative to the vertical than the tooth flank that is positioned in each case on the outside, i.e. positioned facing the joint of the adjacent rack and pinion rods. This joint-side tooth flank, for its part, is formed identically to the tooth flanks of all the internal rack and pinion teeth. As a result of the unsymmetrical configuration of the end teeth, the tooth pitch of the rack and pinion rod, in each case between two end-side rack and pinion teeth or end teeth of adjacently positioned rack and pinion rods, is increased somewhat over the standard tooth pitch, thus allowing, in particular in the case of rack and pinion rods positioned in an angled manner in relation to one another, the engagement ratios between the driving sprocket on the one hand and the rack and pinion rod on the other hand to be improved and constraints between the mutually meshing tooth flanks to be avoided.
Increasing wear to the tooth shape of the driving sprocket can occur in places after a long operating period in the case of a rack and pinion arrangement according to DE 197 46 360 A1 too. The teeth of the toothed wheel drive are basically subject to higher loads than the rack and pinion teeth, as the teeth of the driving sprockets have to transmit the forces of advancement and each individual tooth enters into tooth engagement with a rack and pinion tooth much more frequently than an individual rack and pinion tooth is loaded as a whole. In the generic rack and pinion rods with asymmetrical end teeth, in-depth analyses have shown that increased wear can occur in particular when the jump from the pitch of one end tooth to that of the other end tooth is at the maximum on account of the play, which is necessarily provided, between adjacent rack and pinion rods. Although the narrower configuration of the adjacently positioned end teeth still allows the driving sprocket to roll off via the end tooth, jamming could occur and led to increased wear at the tips of the teeth. Wear, for example on account of jamming, can lead in particular to sharpening of the teeth of the driving sprocket, thus afterwards then also causing increased wear to the guide shoes with which the shearer loader is guided on the rack and pinion rods.